1. Field of the Invention
The present invention relates to a method for measuring the concentration of chelated ferric ions in a solution, especially in a process solution.
2. Description of the Prior Art
There are a number of chemical processes requiring efficient, fast and reliable methods of measuring the concentration of chelated ferric ions in process solutions. One of such processes involves the removal of hydrogen sulfide (H.sub.2 S) from a gaseous stream by contacting the stream with a polyvalent metal chelate solution, e.g., iron chelate solution, to oxidize the H.sub.2 S to elemental sulfur, e.g., see Snavely et al EPA No. 82306861.4, filed Dec. 22, 1982, published under Publication No. 0 086 919 on Aug. 31, 1983, the entire contents of which are incorporated herein by reference.
In the method of Snavely et al, hydrogen sulfide is selectively removed from a gas containing carbon dioxide (CO.sub.2) and the H.sub.2 S in an oxidation-reduction system by contacting the hydrogen sulfide-containing gas stream with a solution of a polyvalent cation, such as iron, complexed with a chelating agent, such as ethylenediaminetetraacetic acid or sodium salt thereof. In such a process, iron in the ferric state (Fe.sup.+++) oxidizes the hydrogen sulfide to sulfur, the iron is reduced to the ferrous state (Fe.sup.++), and the solution is subsequently regenerated by aeration to convert the iron to the ferric state. The sulfur is recovered from the solution by froth flotation.
However, in spite of the cyclic nature of the process and its regeneration cycle, the iron/chelating agent complex degrades causing iron to precipitate, thereby lowering the efficiency of the hydrogen sulfide removal process. Accordingly, iron-chelate complex or free chelating agent must be added to the process loop to maintain a constant level of reactive iron available to remove the H.sub.2 S. The amount of iron must be maintained within relatively narrow limits (e.g., 2500 ppm) depending on the composition of the incoming gas stream and the pH of the process solution. Several methods have been proposed heretofore to monitor the concentration of iron chelate in process solutions. Such methods include analytical methods, e.g., potentiometric, photometric, precipitation analysis and dye endpoint titrations. Some of these methods perform relatively well in relatively physically clean solutions of pure compounds, but they are not sufficiently reliable in process solutions containing a multitude of potentially interfering ions, e.g., thiosulfate (S.sub.2 O.sub.3.sup.=), sulfate (SO.sub.4.sup.=), sodium (Na+), carbonate (CO.sub.3.sup.=), sulfide (S.sup.=) ions. Such process solutions may also be deeply colored. They also may contain suspended solids, such as sulfur (S.degree.) and iron hydroxides, [Fe(OH).sub.x ], wherein X=2 or 3. The coloration and the suspended solids may also interfere with the efficacy of the previously-used iron chelate quantifying methods.
Accordingly, it is a primary object of the present invention to provide a method of determining the concentration of a ferric ion in process solutions.
Additional objects of the invention will become apparent to those skilled in the art from the following specification and the attached claims.